Wow, what a great bunch of ideas in this thread! Here's a few of my own mods:
Lots of my mods are related to storage such as removing the diagonal divider under the bath sink and strapping the pipes to one side, making more room for towels and such:

Removed another divider in front of the wheelwell which gave probably 4 more sq. ft. storage in one cupboard:

There was a fake front on the top drawer of the kitchen drawers -- it didn't actually open. Fixed that problem, but it could only be half depth because of the sink:

I don't have the before pic but I replaced the HUGE cd player with a car-type cd/radio. I've got a door ordered for the opening that was created:

The area under the fridge was inaccessible -- put a door in where we now store all the shoes!

I've no welding skills so I bolted together some underframe storage for the portable sewer cart and an under-bed type storage container for outdoor sort of stuff. The trays tilt down on the opposite edge of the trailer.

I've some mod's for electronics: The solar panel has adjustable aluminum legs to tilt toward the sun. The "shore" power cord is used to plug the trailer in when power is available or acts to connect my solar panel to the batteries otherwise. Gives me about 25' of play to position the panel directly into the sun where possible. There's a switch barely visible to isolate each battery individually or tie them together or cut them completely off. The trailer only came with one battery tray so I had to mod that as well to fit two batteries.
